1. Field of the Invention
This invention relates generally to surveillance air samplers for sampling particulates in ambient air, e.g., to demonstrate local compliance with EPA PM-10 regulations by municipalities and industry, and, more particularly, to an improved particle impactor assembly for size selective fractionating of particulates collected over long periods of time in high volume air samplers.
2. History of the Prior Art
Surveillance air samplers are widely used to monitor particulate air pollution, in aerosol research studies in general, and for safety monitoring in the vicinity of industrial plants that process radioactive materials. Government agencies, especially the United States Environmental Protection Agency (EPA, hereinafter) specify various particulate sampling criteria, generally formulated in terms of the effective aerodynamic diameter of the collected particulates in air flow passed through an air sampler. Likewise, the U.S. Department of Energy (DOE, hereinafter) has also enacted guidelines for DOE licensed nuclear facilities, e.g., 40 CFR 61, part H, due to the highly stringent health considerations for airborne radio nucleides.
In 1984, the EPA developed tentative PM-10 (less than 10 micron particle diameter) criteria for sampling the hazardous fraction of airborne dust, to regulate pollution that comprises particles small enough to be readily deposited in the human respiratory system. The aforementioned DOE guidelines, however, are also concerned with the recovery of particles larger than 10 micron diameters for analysis.
As persons skilled in the art will appreciate, all conventional air samplers include elements, e.g., screens, to keep out airborne insects, relatively large particles blown about during stormy weather, water in the form of raindrops or snowflakes, and the like. Basically, such conventional air samplers convey the air, after such jetsam has been extracted, generally through a plurality of apertures in an inlet element, to an impactor surface substantially normal to the particulate-laden air flow or to a cyclone for collection of particles for subsequent analysis. The smallest particles are typically captured on a fine filter surface.
For many analytical purposes, it is highly desirable to have at least the fine particulate matter fairly uniformly distributed over the filter element for X-ray analysis.
It is also known to apply a sticky coating, e.g., a thin layer of a substance such as petrolatum, commercially available as Vaseline (.TM.), on the impacted surface to capture the larger particles which otherwise may tend to bounce off. Successive collection stages are often utilized in the process to fractionate the particles by size.
U.S. Pat. No. 4,461,183, to Wedding, discloses an aerosol sampler inlet structure with performance characteristics that allow collection of particles small enough to be inhaled by humans (those having aerodynamic diameters less than 10 microns) independent of the sampling conditions of wind speed and direction.
U.S. Pat. No. 4,321,822, to Marple et al, and U.S. Pat. No. 4,211,116, to Pilat et al, are two examples of multistage sampling apparatuses in which different stages capture particles in different size ranges. The Marple et al apparatus includes an element in which apertures ara disposed in spiral configurations, with the impacted surface rotated adjacent thereto so that the deposit of collected dust is essentially uniformly distributed.
U.S. Pat. No. 3,518,815, to McFarland et al, discloses an apparatus in which particulate-laden air is provided through a plurality of elongate slots to a rotating impactor surface.
U.S. Pat. No. 4,038,057, to Roth discloses a closed circuit air sampler in which air is conveyed through pluralities of radially disposed apertures to an impactor surface enclosed in a frangible container, so that removal of the deposited dust requires destruction of the other structure to prevent reuse and possible contamination, deliberate or accidental, in successive uses of the device.
U.S. Pat. No. 3,681,973, to Ludwig, discloses a sampler structure mounted for rotation in a plane perpendicular to the wind direction and having an entrance slot communicating with a hollow aerodynamically shaped chamber lined at its internal surface with a collection paper for collecting the particles along the length thereof, the impingement points of differently sized particles being determined by the trajectories that the particles follow within the chamber, depending on a combination of wind speed, particle diameter, and centrifugal force related to the speed with which the sampler slot is rotated.
Although known air samplers typified by the ones discussed above are capable of separating particulates in the below 10 micron range from those that are larger, they are expensive, complicated to operate, pose problems in handling of the separate collection elements, and do not allow long term operation, e.g., for periods extending for up to four weeks at a time.
There is, therefore, a long felt and serious need for an improved surveillance air sampler for sampling ambient air for long periods of time, which is relatively inexpensive and easy to operate, which is readily adaptable to separate particles having aerodynamic diameters larger than 10 microns from those that are smaller, and which allows for easy recovery of both fractions.